What is extension method in C#?

Answer:

This is a new feature in C# 3.0. This method allows us to add a new static method to the existing classes.

For example, if we want to validate an Email address using the static method of the string class (built-in) class, we can add an extension method like this.

public static bool IsValidEmail(this string input)

        {

            Regex regEx = new Regex(@"\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*");

            return regEx.IsMatch(input);

        }

Notice the parameter passed to above method. Here "this" indicates the string class. Now, we can use above extension method like this.

string myEmail = "me@me.com";

bool IsValidEmailAddress = myEmail.IsValidEmail();

In this case as the format of the email id in myEmail variable is correct so IsValidEmailAddress will be true.